Optical pickup unit having electro-optical element and information recording and reproduction apparatus

ABSTRACT

An optical pickup unit includes a light source emitting a light beam, an objective lens focusing the light beam onto an information recording medium, a light detection part receiving the light beam reflected from the information recording medium, and a light blocking part selectively blocking a part of the light beam with respect to a radial direction. The light blocking part is provided in an optical path of the light beam centered on an optical axis.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to optical pickup units andinformation recording and reproduction apparatuses, and moreparticularly to an optical pickup unit recording information on andreproducing information from information recording media and aninformation recording and reproduction apparatus including such anoptical pickup unit.

2. Description of the Related Art

Disk-type optical information recording media widely used nowadays arecompact disks (CDs) such as CD-ROMs, CD-Rs, and CD-RWs and digitalversatile disks (DVDs) such as DVD-ROMs, DVD-Rs, and DVD-RWs. Some CDshave realized a recording density of 650 MB. The DVDs are larger incapacity than the CDs, but have yet to satisfy the demands of users interms of capacity. In this context, a so-called multilayer disk, a diskformed of a plurality of recording layers instead of a single recordinglayer, has been developed for achieving higher recording density.

In such an information recording medium of multiple recording layers,the recording layers are required to be separated from each other bytens of micrometers (μm) or more so that information may be recorded onand reproduced from each recording layer independently. However,distances from an objective lens to the recording layers are differentso that spherical aberration occurs in recording layers out of anoptimum position. That is, in such recording layers, the position of thefocus of marginal rays (rays entering the periphery of a lens away fromthe optical axis thereof) entering a lens 90 shown in FIG. 1 is deviatedin the direction of the optical axis from the position of the focus ofparaxial rays (rays entering the central part of a lens) entering thelens 90.

FIG. 2 is a diagram for illustrating focusing of a light beam in thecase of reproducing information from a multilayer disk by a singleconventional optical pickup unit. In the case of reproducing informationfrom a first recording layer 101 a of a multilayer disk 101, the lightbeam is focused on the first recording layer 101 a through an objectivelens of the optical pickup unit at a position indicated by 100 a asshown on the left side in FIG. 2. In the case of reproducing informationfrom an n^(th) recording layer 101 n, which is farther away from a disksubstrate surface 101 s than the first recording layer 101 a is, thelight beam is focused on the n^(th) recording layer 101 n through theobjective lens at a position indicated by 100 b as shown on the right inFIG. 2.

If the optical pickup unit is optimized for focusing the light beam intoa spot on the first recording layer 101 a to reproduce the informationtherefrom, the optical pickup unit has no problem in reproducing theinformation from the first recording layer 101 a. However, when theobjective lens of the optical pickup unit gets closer to the disksubstrate surface 101 s of the multilayer disk 101 to reproduce theinformation from the n^(th) recording layer 101 n, and the light beam isfocused into a spot on the n^(th) recording layer 101 n, sphericalaberration occurs due to an interlayer thickness between the first andn^(th) recording layers 101 a and 101 n. As a result, the spot formed onthe n^(th) recording layer 101 n is larger in diameter than the spotformed on the first recording layer 101 a as shown in FIG. 2.

In order to solve this problem, it is necessary to develop a new opticalhead control technology of performing aberration correction by measuringthe amount of spherical aberration of a beam spot. Japanese Laid-OpenPatent Application No. 2000-155979 discloses an aberration detectiondevice as means for solving this problem.

FIG. 3 is a diagram showing a configuration of the aberration detectiondevice. As shown in FIG. 3, a light beam emitted from a light source 201and reflected from an optical disk 206 is split by a half mirror 202 tobe divided into a light beam passing through a specific region and alight beam passing through the other regions by a hologram 209. Thelight beam passing through the specific region is deflected by thehologram 209 to be received by a plurality of photodetectors 207 so thatthe photodetectors 207 obtain respective signals. The obtained signalsare compared so that an aberration is detected. The detected aberrationis transmitted via an aberration signal processing circuit 208 to anaberration correction device 204 so that the aberration correctiondevice 204 can be driven in real time based on the aberration so as toreduce the aberration of the optical system. In FIG. 3, referencenumerals 203 and 205 denote a collimator lens and an objective lens,respectively.

The assembly conditions of a light-receiving element in the opticalpickup unit are extremely strict so that the light-receiving element isrequired to be provided with an accuracy of a few micrometers or less.This reduces yield rate, thereby affecting the cost to a considerableextent. Further, the aging and temperature characteristics of thelight-receiving element are subject to change. Therefore, it isdesirable that a light-receiving element pattern be simple. However,according to the technology disclosed in Japanese Laid-Open PatentApplication No. 2000-155979, an incident light beam for aberrationdetection is split into a plurality of beams by a hologram so that aplurality of light-receiving elements detect a given one of the splitbeams. Such a configuration, where each split beam is focused into asmall spot and a plurality of light-receiving elements detect a givenone of the split beams, is complicated and may impair the stability ofthe optical pickup unit. This may reduce yield rate, thereby incurringan increase in cost.

The optical pickup unit is known as a device for recording informationon and reproducing recorded information from an information recordingmedium. The ratio of the intensity of a light beam focused onto theinformation recording medium for recording to that for reproductionranges from 5:1 to 15:1. Generally, the emission power of a light sourceswitches between recording time and reproduction time substantially inaccordance with the ratio. However, in some semiconductor lasers, noisecharacteristics worsen as an output lowers.

That is, part of a light beam emitted from a semiconductor laser returnsto the semiconductor laser as a returning light. A different resonatorother than the semiconductor laser is formed between the returning lightand the information recording medium. As a result, the state ofoscillation of the semiconductor laser becomes unstable so that theoutput of the semiconductor laser includes noise. Further, noise is alsogenerated in the output of the semiconductor laser by the operation ofthe semiconductor laser or a variation in environmental temperature.When the light-emission power of the semiconductor laser is far above athreshold current level to reach tens of milliwatts (mW), the lightemission of the semiconductor laser is stable, being hardly affected bydisturbances. However, when the light-emission power of thesemiconductor laser is around the threshold current level, thelight-emission power is affected by disturbances including those causedby the above-described returning light, so that variations are caused inthe light-emission power.

Accordingly, in the case of recording information on (writinginformation to) the information recording medium or erasing informationrecorded thereon, the light emission of the semiconductor laser ishardly affected by disturbances since the light-emission power of thesemiconductor laser is far above the threshold current level. However,in the case of reproducing (reading out) information from theinformation recording medium, the light-emission power of thesemiconductor laser is normally set to a low level so that thesemiconductor laser emits the laser beam with a power of a fewmilliwatts slightly over the threshold current level, for instance, apower of five milliwatts. In this case, therefore, the semiconductorlaser is especially subject to the returning light to be unstable inemitting the laser beam. Accordingly, the signal is deteriorated bynoise caused in the output of the semiconductor laser.

Japanese Laid-Open Patent Application No. 9-27141 discloses an opticalpickup unit to solve this problem. According to this optical pickupunit, an electro-optical element capable of controlling transmittance oflight is provided in an optical path from a light source to a recordingmedium. The electro-optical element controls transmittance for lightemitted from the light source to a low rate (value) at a time ofreproducing information, and to a high rate (value) at a time ofrecording information.

Further, in an optical pickup unit, light emitted from a semiconductorlaser is focused onto a surface of an information recording mediumthrough a focus optical system, and a reflected light from theinformation recording medium is directed through a detection opticalsystem to a light-receiving element. Generally, the detection signal(electric current signal) of the light-receiving element of the opticalpickup unit is converted into a voltage signal by a current-voltageconversion amplifier housed in the optical pickup unit to be output to-asignal processing circuit.

If the amplitude level of the signal output through the current-voltageconversion amplifier is too low, a problem is caused in informationreproduction. Therefore, such a configuration is employed that the gainof the current-voltage conversion amplifier is switchable so that theoutput amplitude level thereof falls within a proper range. However, inrecent years, it has been required for the optical pickup unit toaccommodate a plurality of conditions so as to be suitable for a varietyof types of information recording media and various recording andreproduction conditions. Accordingly, in order to perform informationrecording, reproduction, and erasure in compliance with a plurality oftypes of optical disks, such as a CD-RW (compact disk rewritable), aCD-DA (compact disk digital audio), a CD-ROM, and a CD-R (compact diskrecordable), as performed by an optical information recording andreproduction apparatus disclosed in Japanese Laid-Open PatentApplication No. 10-255301, it is necessary to make such a gain switchingcircuit shown in FIG. 4 suitable for more combination patterns of disktypes and light powers. Consequently, the number of resistors employedin the gain switching circuit increases so that the speed of response ofa signal is reduced.

In addition, recently, it has been proposed to increase the numericalaperture (NA) of an objective lens for focusing a light beam onto aninformation recording medium so as to achieve high recording density.This is because the diameter of a beam spot can be reduced by using anobjective lens of a large NA. However, as the NA increases, the rate ofincrease of aberration also increases. That is, spherical aberration,whose primary cause is a substrate thickness error in the informationrecording medium, is proportional to the NA to the fourth power, andcoma, whose primary cause is the inclination of the informationrecording medium to the optical axis, is proportional to the cube of theNA.

Japanese Laid-Open Patent Application Nos. 10-20263, 9-128785,11-259892, and 2000-155979 disclose techniques to correct and detectwavefront aberration (spherical aberration, coma, and astigmatism),which techniques are known as prior art for solving the above-describedproblems.

However, optical pickup units having such configurations as disclosed inthe above-described references need to have their costs reduced bycomponent sharing and their assembly processes simplified.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean optical pickup unit and an information recording and reproductionapparatus in which the above-described disadvantages are eliminated.

A more specific object of the present invention is to provide anoptical,pickup unit that can stably perform spherical aberrationdetection, being made less subject to the effects of variations in itsaging and temperature characteristics by providing a light-blocking partusing a liquid crystal element in a substantially parallel optical pathwithout increasing the number of light-receiving elements as in theconventional optical pickup unit, the optical pickup unit also realizingcost reduction by reducing the number of components and the number ofassembly processes.

Another more specific object of the present invention is to provide anoptical pickup unit including an electro-optical element dividing theregion of a light beam passing therethrough so as to be capable ofperforming stable control without its light source being affected bynoise at a time of reproduction, processing a light reception signalwithout decreasing response speed, and performing corrections based onwavefront aberration, the optical pickup unit realizing componentsharing, reduction in the number of components, and simplification ofassembly adjustment.

Yet another more specific object of the present invention is to providean information recording and reproduction apparatus including any of theabove-described optical pickup units.

The above-objects of the present invention are achieved by an opticalpickup unit including a light source emitting a light beam, an objectivelens focusing the light beam onto an information recording medium, alight detection part receiving the light beam reflected from theinformation recording medium, and a light blocking part selectivelyblocking a part of the light beam with respect to a radial direction,the light blocking part provided in an optical path of the light beam tobe centered on an optical axis.

The above-described optical pickup unit is capable of detecting raysaround the axis of a beam spot and the rays of the peripheral part ofthe beam spot separately. Therefore, spherical aberration, which is aphenomenon of a difference between focus positions, can be detectedbased on the detection signals of the rays.

Additionally, the above-described optical pickup unit may furthercomprise a control part generating an aberration signal by comparing afirst signal generated based on a first part of the light beam passingthrough a first region of the light blocking part and a second signalgenerated based on a second part of the light beam passing through asecond region of the light blocking part, the first region of the lightblocking part being provided internal to the second region thereof.

Thereby, an amount of correction of spherical aberration can be set byusing the aberration signal.

Additionally, the above-described optical pickup unit may furthercomprise a spherical aberration correction part correcting sphericalaberration based on the aberration signal generated by the control part.

Thereby, the spherical aberration can be corrected based on the amountof correction obtained from the aberration signal.

Additionally, in the above-described optical pickup unit, the lightblocking part and the spherical aberration correction part may form asingle element.

Thereby, the above-described optical pickup unit, which can be realizedwith reduced space, has its number of components reduced so that costreduction thereof can be realized.

The above objects of the present invention are also achieved by aninformation recording and reproduction apparatus including an opticalpickup unit that includes a light source emitting a light beam, anobjective lens focusing the light beam onto an information recordingmedium, a light detection part receiving the light beam reflected fromthe information recording medium, and a light blocking part selectivelyblocking a part of the light beam with respect to a radial direction,the light blocking part provided in an optical path of the light beam tobe centered on an optical axis.

According to the above-described optical pickup unit, rays around theaxis of a beam spot and the rays of the peripheral part of the beam spotcan be detected separately. Therefore, spherical aberration, which is aphenomenon of a difference between focus positions, can be detectedbased on the detection signals of the rays.

The above objects of the present invention are also achieved by anoptical pickup unit performing recording or reproducing information bymaking a light beam emitted from a light source incident on aninformation recording medium, the optical pickup unit including anelectro-optical element switching values of transmittance of a givenregion through which the light beam passes depending on whether theinformation is recorded or reproduced.

Additionally, in the above-described optical pickup unit, theelectro-optical element may be provided subsequent to the light sourcein an optical path between the light source and the informationrecording medium, and the transmittance of the given region of theelectro-optical element may be controlled to a first value at a time ofreproducing the information from the information recording medium and toa second value at a time of recording the information on the informationrecording medium, the first value being smaller than the second value.

Additionally, in the above-described optical pickup unit, theelectro-optical element may include an outer region provided outside thegiven region so that the light beam passing through the outer region hasa phase difference thereof selectively varied in the outer region.

According to the above-described optical pickup unit, stable control canbe performed without noise affecting the light beam passing through thegiven region. Further, the light beam passing through the outer regionhas its phase difference selectively varied so that deterioration of thewave surface of the light beam incident on the information recordingmedium can be suppressed. Thereby, the spot performance of the lightbeam collected by an objective lens can be secured.

Additionally, in the above-described optical pickup unit, the light beampassing through the outer region may have the phase difference thereofselectively varied concentrically in the outer region.

Additionally, in the above-described optical pickup unit, the light beampassing through the outer region may have the phase difference thereofvaried in the outer region step by step in a radial or a tangentialdirection.

Additionally, in the above-described optical pickup unit, the light beampassing through the outer region may have the phase difference thereofvaried in the outer region simultaneously and asymmetrically in radialand tangential directions.

Thereby, wavefront aberration (spherical aberration, coma, andastigmatism) can be suppressed.

The above objects of the present invention are further achieved by aninformation recording and reproduction apparatus recording informationon and reproducing information from an information recording medium, theinformation recording and reproduction apparatus including an opticalpickup unit performing recording or reproducing information by making alight beam emitted from a light source incident on the informationrecording medium, the optical pickup unit including an electro-opticalelement switching values of transmittance of a given region throughwhich the light beam passes depending on whether the information isrecorded or reproduced.

According to the above-described information recording and reproductionapparatus, information recording and reproduction can be stablyperformed.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram for illustrating spherical aberration;

FIG. 2 is a diagram for illustrating focusing of a light beam in a caseof reproducing information from a multilayer disk by a singleconventional optical pickup unit;

FIG. 3 is a schematic diagram showing an optical system of an opticalpickup unit including a conventional aberration detection device;

FIG. 4 is a diagram showing a conventional gain switching circuit;

FIG. 5 is a schematic diagram showing an optical system of an opticalpickup unit according to a first embodiment of the present invention;

FIG. 6 is a plan view of a turning part of the optical pickup unitaccording to the first embodiment, showing transparent electrodepatterns formed thereon;

FIG. 7 is a sectional view of the turning part of FIG. 6;

FIG. 8 is a plan view of a spherical aberration correction part of theoptical pickup unit according to the first embodiment, showingtransparent electrode patterns formed thereon;

FIGS. 9A through 9G are diagrams for illustrating correction ofspherical aberration in an information recording and reproductionapparatus including the optical pickup unit according to the firstembodiment;

FIG. 10 is a diagram for illustrating a configuration of the opticalpickup unit of the first embodiment in which another sphericalaberration correction element is employed;

FIG. 11 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a second embodiment of thepresent invention;

FIG. 12 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a third embodiment of thepresent invention;

FIG. 13 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a fourth embodiment of thepresent invention;

FIG. 14 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a fifth embodiment of thepresent invention;

FIG. 15 is a perspective view of an information recording andreproduction apparatus according to a sixth embodiment of the presentinvention;

FIG. 16 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a seventh embodiment ofthe present invention;

FIG. 17 is a sectional view of a first electro-optical element (liquidcrystal cell) of the optical pickup unit of FIG. 16;

FIG. 18 is a diagram showing a relationship between transmittance andapplied voltage in the liquid crystal cell of FIG. 17;

FIG. 19 is a diagram showing a transmittance change region (a firstregion) and an aberration correction region (a second region) of thefirst electro-optical element of FIG. 17;

FIGS. 20A through 20C are diagrams showing spherical aberration, coma,and astigmatism, respectively;

FIG. 21 is another diagram showing the spherical aberration;

FIG. 22A is a diagram showing a phase of a light beam at a time ofoccurrence of the spherical aberration, and FIG. 22B is a diagramshowing a pattern of an electro-optical element for selectivelytransmitting the light beam;

FIG. 23A is a diagram showing a section of the spherical aberration anda phase difference for correcting the spherical aberration, and FIG. 23Bis a diagram showing the spherical aberration after correction;

FIGS. 24A through 24C are diagrams showing patterns for correcting thespherical aberration, coma, and astigmatism, respectively, in theelectro-optical element;

FIG. 25A is a diagram showing a phase of the light beam at a time ofoccurrence of the coma, and FIG. 25B is a diagram showing a pattern ofthe electro-optical element for selectively transmitting the light beam;and

FIG. 26A is a diagram showing a section of the coma and a phasedifference for correcting the coma, and FIG. 26B is a diagram showingthe coma after correction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the accompanyingdrawings, of embodiments of the present invention.

FIG. 5 is a schematic diagram showing an optical system of an opticalpickup unit according to a first embodiment of the present invention.The optical system of the optical pickup unit of FIG. 5 includes amultilayer information recording medium 1, a semiconductor laser 2emitting a p-polarized laser beam, a collimator lens 3 collimates thelaser beam emitted from the semiconductor laser 2 into parallel rays, abeam splitter 4 letting through the laser beam from the collimator lens3 and deflecting a reflected light from the multilayer informationrecording medium 1, a deflection mirror 5 deflecting the laser beam, anobjective lens 6 gathering the laser beam incident thereon with itsoptical characteristics such as numerical aperture and sphericalaberration being optimized for one of the layers of the multilayerinformation recording medium 1, a turning part 7 that selectively turnsthe plane of polarization of the reflected light from the multilayerinformation recording medium 1 deflected by the beam splitter 4, apolarizing plate 8 transmitting the p-polarized reflected light, adetection lens 9 gathering the reflected light from the polarizing plate8, a light-receiving element 10 receiving the reflected light to outputa tracking signal, a focus signal, and a reproduction signal, aspherical aberration detection part 11 detecting spherical aberrationfrom the output of the light-receiving element 10, a sphericalaberration correction part 12, and a spherical aberration controlcircuit 13 driving and controlling the spherical aberration correctionpart 12 based on the output of the spherical aberration detection part11.

As shown in FIG. 6, the turning part 7 is formed of a liquid crystalelement composed of a central region A and a peripheral region B.Voltage is applied to a selected one of the central region A and theperipheral region B so that the direction of polarization of the lightbeam passing through the central region A or the peripheral region B canbe changed by the applied voltage.

As shown in FIG. 7, the turning part 7 has a twisted nematic (TN) liquidcrystal 7 a sandwiched, in the direction of the optical axis of theturning part 7, between two glass plates 7 b and 7 c with transparentelectrodes. When a control voltage (control signal) of zero volts isapplied (that is, no voltage is applied) between an electrode of theupper glass plate 7 b (hereinafter referred to simply as an upperelectrode) and an electrode of the lower glass plate 7 c (hereinafterreferred to simply as a lower electrode), the plane of polarization ofthe laser beam is turned 90° by the turning part 7. On the other hand,when a voltage higher than a threshold voltage, such as a controlvoltage (control signal) of five volts, is applied between the upper andlower electrodes, the laser beam passes through the turning part 7 as itis without its plane of polarization being turned.

Next, a description will be given of an operation of the turning part 7.

First, a voltage higher than the threshold voltage is applied betweenthe upper and lower electrodes of the central region A of the turningpart 7, while a voltage of zero volts is applied to the peripheralregion B. Thereby, the light beam passing through the turning part 7 hasits plane of polarization remaining unchanged in its part passingthrough the central region A and turned 90° in its part passing throughthe peripheral region B. Under this setting condition, the p-polarizedlaser beam is emitted from the semiconductor laser 2 and reflected fromthe multilayer information recording medium 1 to be incident on theturning part 7 through the beam splitter 4.

Since the voltage higher than the threshold voltage is applied to thecentral region A, the light beam passes through the central region A toremain p-polarized with its plane of polarization remaining unchanged.On the other hand, since the voltage of zero volts is applied to theperipheral region B, the light beam has its plane of polarization turnedby 90° in the peripheral region B so that an s-polarized light isemitted therefrom. Thereafter, the laser beam is incident on thepolarizing plate 8. Since the polarizing plate 8 lets through ap-polarized light, the laser beam from the central region A of theturning part 7 passes through the polarizing plate 8, while thes-polarized light beam from the peripheral region B of the turning part7 is prevented from passing through the polarizing plate 8.

That is, the laser beam from the central region A of the turning part 7passes through the polarizing plate 8 as it is to be incident on thedetection lens 9, but the laser beam from the peripheral region B of theturning part 7 is blocked off by the polarizing plate 8 and is notincident on the detection lens 9. The laser beam from the central regionA of the turning part 7 is collected by the detection lens 9 to bereceived by the light-receiving element 10 so that a focus position isoutput from the light-receiving element 10.

Next, the voltage applied to the central region A and the voltageapplied to the peripheral region B are switched so that the voltage ofzero volts is applied between the upper and lower electrodes of thecentral region A and the voltage higher than the threshold voltage isapplied to the peripheral region B. In this case, contrary to theprevious case where the voltage higher than the threshold voltage isapplied to the central region A and the voltage of zero volts is appliedto the peripheral region B, the laser beam from the peripheral region Bof the turning part 7 is incident on the light-receiving element 10. Afocus position is output from the light-receiving element 10 based onthe incident light beam.

The signals of focus position (focus position signals) output from thelight-receiving element 10 are input to the spherical aberrationdetection part 11. The spherical aberration detection part 11 obtains adifference between the focus position signals output from thelight-receiving element before and after the switching of the voltagesapplied to the turning part 7. The spherical aberration detection part11 outputs the difference to the spherical aberration control circuit13.

Here, spherical aberration is a phenomenon that the focus position ofrays around the axis of a beam spot (paraxial rays) is different fromthe focus position of rays of the peripheral part of the beam spot(marginal rays). Therefore, an amount of spherical aberration can beobtained from a difference between the focus positions by detecting thefocus positions individually by separating the paraxial rays from themarginal rays. According to the first embodiment, the sphericalaberration detection part 11 is capable of detecting the focus positionsof the two light beams separately. The spherical aberration controlcircuit 13 generates a control signal proportional to the amount ofspherical aberration based on the difference between the two focuspositions. The spherical aberration control circuit 13 drives andcontrols the spherical aberration correction part 12 based on thecontrol signal so that the spherical aberration is corrected.

Next, a more detailed description will be given of the sphericalaberration correction part 12.

The spherical aberration correction part 12 includes two glasssubstrates with transparent electrodes and liquid crystal moleculessandwiched between the glass substrates. An upper one of the two glasssubstrates (hereinafter, an upper glass substrate) has concentricelectrode patterns formed thereon of transparent electrodes as shown inFIG. 8. Electrodes are formed on a lower one of the two glass substrates(hereinafter, a lower glass substrate) so as to oppose the concentricelectrode patterns formed on the upper glass substrate. The concentrictransparent electrode patterns may be formed on the lower glasssubstrate instead of the upper glass substrate.

When driving voltages are applied to the transparent electrodes, theliquid crystal molecules are aligned in accordance with electric fieldsgenerated by the applied voltages. Thereby, a refractive indexdistribution can be set as desired in a section of the light beampassing through the spherical aberration correction part 12 whichsection is perpendicular to a direction in which the light beam travels.Accordingly, the wave surface of the light beam can be divided intoregions so that the phase of the wave surface can be controlledindependently in each divided region. That is, the spherical aberrationcorrection part 12 is employable for changing a refractive index.

Therefore, by variably controlling the voltage applied to each of thetransparent electrode patterns formed on the upper or lower glasssubstrate, spherical aberration caused by a distance or an interlayerthickness between the recording layer for which the optical pickup unitis optimized and a recording layer from which information is to bereproduced can be corrected. Thereby, an optical pickup unitautomatically correcting spherical aberration can be realized.

Next, a description will be given, with reference to FIGS. 9A through9G, of correction of spherical aberration in an information recordingand reproduction apparatus including the optical pickup unit of thefirst embodiment. In each of FIGS. 9A through 9C, 9E, and 9F, a point Oon a horizontal axis corresponds to an optical axis, and D–D′ representspositions on a straight line that perpendicularly crosses the opticalaxis at the point O. For instance, D–D′ represents the pupil surface ofthe objective lens. Further, a vertical axis L represents an amount ofspherical aberration. FIG. 9D is a diagram showing a liquid crystalpanel on which three concentrically divided electrodes 14 through 16 areformed. FIG. 9G is a diagram showing electrode patterns in the case offorming five divided electrodes on the liquid crystal panel.

FIG. 9A is a diagram showing a distribution pattern of sphericalaberration caused by the interlayer thickness between the recordinglayer for which the optical pickup unit is optimized and the recordinglayer from which information is to be reproduced. The sphericalaberration shown in FIG. 9A is obtained by converting sphericalaberration occurring on the recording layer into spherical aberration onthe pupil surface of the objective lens by ray tracing.

Normally, the laser beam is focused into a circular beam spot so thatspherical aberration varies in a radial direction. If the interlayerthickness increases, the two peaks of the spherical aberration shown inFIG. 9A become higher. In the case of occurrence of such sphericalaberration, the liquid crystal panel of FIG. 9D is controlled so thatvoltage is applied to the electrode 15 so as to provide, as shown inFIG. 9B, a phase difference canceling the spherical aberration of FIG.9A to a light beam passing through the part of the electrode 15, whilevoltages are applied to the electrodes 14 and 16 so that light beamspass through the parts of the electrodes 14 and 16 as they are withoutany phase differences being provided thereto.

Thus, by applying the different voltages to the concentrically dividedelectrodes 14 through 16 formed on the liquid crystal panel, correctionas shown in FIG. 9B is performed on the spherical aberration of FIG. 9Aso that residual spherical aberration can be minimized as shown in FIG.9C. Thereby, the diameter of the beam spot can be reduced. FIGS. 9E and9F are diagrams showing a correction to the spherical aberration and theresult of the correction, respectively, in the case where the fiveconcentrically divided electrodes are formed on the liquid crystal panelas shown in FIG. 9G. By controlling the liquid crystal display of FIG.9G so that different voltages are applied to the five divided electrodesso as to provide phases canceling the spherical aberration to lightbeams passing through given ones of the five electrodes as shown in FIG.9E, a more precise correction can be made to the spherical aberration asshown in FIG. 9F. The residual spherical aberration is further reducedin FIG. 9F compared with FIG. 9C, so that the diameter of the beam spotcan be further decreased.

According to the optical pickup unit thus configured according to thefirst embodiment, which unit employs the spherical aberration detectionpart 11 and the spherical aberration correction part 12, an amount ofspherical aberration can be measured by the spherical aberrationdetection part 11 so that the peripheral region of the laser beam can beoptimized by the spherical aberration correction part 12 even if theperipheral region of the laser beam is not focused on a recordingsurface of the multilayer information recording medium 1 by the effectof the spherical aberration in the case of reproducing information froma recording layer other than the recording layer for which the objectivelens 6 of the optical pickup unit is optimized. Therefore, recording canbe performed on the multilayer information recording medium 1 with highaccuracy and a high-quality signal can be reproduced from the multilayerinformation recording medium 1. The same effects can be produced by theinformation recording and reproduction apparatus including theabove-described optical unit according to the first embodiment.

In the optical pickup unit of FIG. 5, the turning part 7 and thepolarizing plate 8 may be formed as a single unit. This reduces thenumber of assembly steps, thus realizing cost reduction. Further, inthis case, the optical pickup unit can be further downsized bydepositing a polarizing film on the turning part 7 instead of attachingthe polarizing plate 8 thereto.

Further, the polarizing plate 8 of FIG. 5 may be replaced by anotheroptical device for polarization selection, such as a polarization beamsplitter or a polarization hologram.

Furthermore, the spherical aberration correction part 12 of the opticalpickup unit of FIG. 5 is not limited to a liquid crystal element, whichis employed in the first embodiment.

FIG. 10 is a diagram for illustrating a configuration of the opticalpickup unit of the first embodiment in which a movable lens is employedto perform spherical aberration correction.

The optical pickup unit of FIG. 10 includes an objective lens 20 and aspherical aberration correction part 21. In FIG. 10, the same elementsas those of FIG. 5 are referred to by the same numerals, and adescription thereof will be omitted.

In the configuration of FIG. 10, the objective lens 20 composed of twoaspherical lenses 20 a and 20 b replaces the objective lens 6 of FIG. 5,and the spherical aberration correction part 21 having a distanceadjustment mechanism varying a distance between the aspherical lenses 20a and 20 b replaces the spherical aberration correction part 12 of FIG.5. The distance adjustment mechanism of the spherical aberrationcorrection part 21 is provided between the aspherical lenses 20 a and 20b.

A piezoelectric element is applicable as the distance adjustmentmechanism. The distance between the aspherical lenses 20 a and 20 bbecomes greater by applying a higher voltage to the piezoelectricelement and smaller by applying a lower voltage thereto. Sphericalaberration can be corrected by generating such a variation in voltagebased on the output of the spherical aberration detection part 11.

According to the configuration of FIG. 10, spherical aberration on aninformation recording layer can be reduced so that the optical pickupunit has good recording and reproduction characteristics.

In the optical pickup unit of FIG. 10, an electromagnetically drivenactuator or motor may be used as the distance adjustment mechanisminstead of the piezoelectric element. An actuator driven by ultrasonicwaves is also employable instead of the piezoelectric element. Further,a group of two convex lenses or a combination of an aspherical lens anda spherical lens may be employed as the objective lens 20 instead of thetwo aspherical lenses 20 a and 20 b.

FIG. 11 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a second embodiment of thepresent invention. The optical pickup unit of FIG. 11 includes apolarization beam splitter 30 and a turning part 31. In FIG. 11, thesame elements as those of FIGS. 5 and 10 are referred to by the samenumerals, and a description thereof will be omitted.

The optical pickup unit of the second embodiment is different from thatof the first embodiment in the following points:

(a) The polarization beam splitter 30 replaces the beam splitter 4 ofthe first embodiment.

(b) The turning part 31, which replaces the turning part 7 of the firstembodiment, is provided in the optical path between the polarizationbeam splitter 30 and the objective lens 20.

(c) The turning part 31 includes a functional element corresponding to a¼ wave plate, while the turning part 7 of the first embodiment isemployed as a functional element corresponding to a 1/2 wave plate (thatis, the turning part 7 turns a plane of polarization 90° when thevoltage is applied to the turning part 7, and does not turn the plane ofpolarization when no voltage is applied thereto).

(d) The polarizing plate 8 of the first embodiment is not employed inthe second embodiment.

The ¼ wave plate circularly polarizes the linearly polarized laser beamemitted from the semiconductor laser 2 so that the circularly polarizedlight beam is incident on the multilayer information recording medium 1.On the other hand, the ¼ wave plate linearly polarizes a reflected lightbeam from the multilayer information recording medium 1, which lightbeam is circularly polarized in the reverse direction, into a light beamwhose plane of polarization is perpendicular to that of the light beamemitted from the semiconductor laser 2 toward the multilayer informationrecording medium 1. Unlike the non-polarization beam splitter 4 of thefirst embodiment, the polarization beam splitter 30 is a polarizingelement of ‘1’/‘0’ that transmits a p-polarized laser beam and reflectsan s-polarized laser beam. The second embodiment employs the samecomponents as the first embodiment except those mentioned above.

Like the configuration shown in FIG. 6, the turning part 31 is formed ofa liquid crystal element composed of the central region A and theperipheral region B. Voltage is applied to a selected one of the centralregion A and the peripheral region B so that the state of polarizationof the light beam passing through the central region A or the peripheralregion B may be changed by the applied voltage.

Like the configuration of FIG. 7, the turning part 31 has a liquidcrystal sandwiched, in the direction of an optical axis, between twoupper and lower glass plates with transparent electrodes. When a voltagelower than a threshold voltage is applied between the upper and lowerelectrodes in the region A or B, the ¼ wave plate function of theturning part 31 is started so that the laser beam is emitted therefromcircularly polarized. On the other hand, when a voltage higher than thethreshold voltage is applied between the upper and lower electrodes, theturning part 31 loses its refractive index anisotropy, causing no phasedifference in the laser beam with Δn=0. Therefore, the laser beam passesthrough the turning part 31 as it is without its plane of polarizationbeing turned.

In the case of recording information on or reproducing information fromthe multilayer information recording medium 1, first, the voltage higherthan the threshold voltage is applied between the upper and lowerelectrodes of the central region A, while the voltage lower than thethreshold value is applied to the peripheral region B. Thereby, thelight beam passing through the turning part 31 has its plane ofpolarization remaining unchanged in its part passing through the centralregion A and circularly polarized in its part passing through theperipheral region B. Under this setting condition, the p-polarized laserbeam is emitted from the semiconductor laser 2 to be incident on theturning part 31.

Since the voltage higher than the threshold voltage is applied to thecentral region A, the incident laser beam passes through the centralregion A as the p-polarized light with its plane of polarizationremaining unchanged. On the other hand, since the voltage lower than thethreshold voltage is applied to the peripheral region B, the laser beamhas its plane of polarization being circularly polarized in theperipheral region B. The laser beam is focused onto the multilayerinformation recording medium 1 with its parts passing through theregions A and B being in different states of polarization. The laserbeam from the central region A is reflected from the multilayerinformation recording medium 1 as the p-polarized light, and is againincident on the turning part 31 to pass therethrough as the p-polarizedlight. On the other hand, the circularly polarized laser beam from theperipheral region B is focused onto the multilayer information recordingmedium 1 to be reflected therefrom circularly polarized in the reversedirection. The reflected light beam is again incident on the turningpart 31 to be converted into an s-polarized light having a polarizationdirection perpendicular to that of the light beam emitted from thesemiconductor laser 2 toward the multilayer information recording medium1. The laser beams from the central region A and the peripheral region Bare incident on the polarization beam splitter 30. The polarization beamsplitter 30 transmits the p-polarized light and reflects the s-polarizedlight. Therefore, the laser beam from the central region A of theturning part 31 passes through the polarization beam splitter 30, whilethe laser beam from the peripheral region B of the turning part 31 isreflected from the polarization beam splitter 30.

That is, the laser beam from the peripheral region B of the turning part31 is incident on the detection lens 9, while the laser beam from thecentral region A of the turning part 31 is not. The laser beam from theperipheral region B of the turning part 31 is gathered by the detectionlens 9 to be received by the light-receiving element 10 so that a focusposition is output therefrom.

Next, the voltage applied to the central region A and the voltageapplied to the peripheral region B are switched so that the voltagelower than the threshold voltage, that is, the voltage of zero volts, isapplied between the upper and lower electrodes of the central region Aand the voltage higher than the threshold voltage is applied to theperipheral region B. In this case, contrary to the previous case wherethe voltage higher than the threshold voltage is applied to the centralregion A while the voltage of zero is applied to the peripheral regionB, the laser beam from the central region A of the turning part 31 isincident on the light-receiving element 10. A focus position is outputfrom the light-receiving element 10 based on the incident light beam.

As in the first embodiment, the spherical aberration control circuit 13drives and controls the spherical aberration correction part 21 based onan amount of spherical aberration obtained from the output focusposition signals thus generated, thereby eliminating the sphericalaberration. In the second embodiment, after the spherical aberration isdetected, the voltage lower than the threshold voltage is applied toeach of the,central region A and the peripheral region B of the turningpart 31 so that the turning part 31 functions as a ¼ wave plate. Asystem employing a combination of the ¼ wave plate and the polarizationbeam splitter 30 is highly advantageous in an information recording andreproduction apparatus having a recording system requiring highefficiency of use of light.

FIG. 12 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a third embodiment of thepresent invention. In FIG. 12, the same elements as those of FIG. 5 arereferred to by the same numerals, and a description thereof will beomitted.

The optical pickup unit of the third embodiment is different from thatof the first embodiment in that the turning part 7 is provided in theoptical path from the semiconductor laser 2 to the multilayerinformation recording medium 1. The turning part 7 of the thirdembodiment is substantially equal to that of the first embodiment. Theturning part 7 of the third embodiment is identical to that of the firstembodiment in being formed of the liquid crystal element composed of thecentral region A and the peripheral region B as shown in FIG. 6 so thatvoltage is applied to a selected one of the central region A and theperipheral region B, thereby changing the direction of polarization ofthe light beam passing through the central region A or the peripheralregion B.

In the case of recording information on or reproducing information fromthe multilayer information recording medium 1, first, a voltage higherthan a threshold voltage is applied between the upper and lowerelectrodes of the central region A, while a voltage of zero volts isapplied to the peripheral region B. Thereby, of the light beam passingthrough the turning part 7, a light beam passing through the centralregion A has its plane of polarization remaining unchanged, while alight beam passing through the peripheral region B has its plane ofpolarization turned 90°. Under this setting condition, the p-polarizedlight beam is emitted from the semiconductor laser 2.

Since the voltage higher than the threshold voltage is applied to thecentral region A, the incident laser beam passes therethrough as thep-polarized light with its plane of polarization remaining unchanged. Onthe other hand, since the voltage of zero volts is applied to theperipheral region B, the laser beam passing therethrough has its planeof polarization turned by 90° to be emitted therefrom as an s-polarizedlight. Then, the laser beam passing through the turning part 7 isreflected from the multilayer information recording medium 1 anddeflected by the beam splitter 4 to be incident on the polarizing plate8. The polarizing plate 8 transmits the p-polarized reflected light.Therefore, the laser beam from the central region A of the turning part7 passes through the polarizing plate 8, while the s-polarized laserbeam from the peripheral region B of the turning part 7 is preventedfrom passing through the polarizing plate 8. That is, the laser beamfrom the central region A of the turning part 7 passes through thepolarizing plate 8 as it is to be incident on the detection lens 9,while the laser beam from the peripheral region B of the turning part 7is prevented from being incident on the detection lens 9. The laser beamfrom the central region A of the turning part 7 is gathered by thedetection lens 9 and received by the light-receiving element 10 so thata focus position is output therefrom.

Next, the voltage applied to the central region A and the voltageapplied to the peripheral region B are switched so that the voltage ofzero volts is applied between the upper and lower electrodes of thecentral region A and the voltage higher than the threshold voltage isapplied to the peripheral region B. In this case, contrary to theprevious case where the voltage higher than the threshold voltage isapplied to the central region A and the voltage of zero volts is appliedto the peripheral region B, the laser beam from the peripheral region BQf the turning part 7 is incident on the detection lens 9. A focusposition is output from the light-receiving element 10 based on theincident light beam.

As in the first embodiment, the spherical aberration control circuit 13drives and controls the spherical aberration correction part 12 based onan amount of spherical aberration obtained from the output focusposition signals thus generated, thereby eliminating the sphericalaberration.

FIG. 13 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a fourth embodiment of thepresent invention. The optical pickup unit of the fourth embodimentincludes a turning and spherical aberration correction part 40 and afocus driver 41 that performs focus control on an actuator (not shown inthe drawing) moving the objective lens 6 in focus and trackingdirections.

The turning and spherical aberration correction part 40 has the glassplates 7 b and 7 c with the transparent electrodes and the TN liquidcrystal 7 a sandwiched, in the direction of the optical axis, betweenthe glass plates 7 b and 7 c as shown in FIG. 7. Further, the glassplate 7 b or 7 c has the concentric transparent electrode patternsformed thereon. Such switching between the central region A and thespherical region B as illustrated in FIG. 6 and such selection of anamount of reduction of the beam spot diameter as illustrated in FIGS. 8and 9A through 9G can be performed by selecting electrode patterns towhich voltage are to be applied.

The multilayer information recording medium 1 is rotated by aninformation recording medium holding part (not shown in the drawing) andan information recording medium rotation control device (not shown inthe drawing). The objective lens actuator is provided in the opticalpickup unit to oppose the multilayer information recording medium 1. Theobjective lens actuator drives the objective lens 6 so that the laserbeam is converged on a recording layer selected based on informationread from the multilayer information recording medium 1 or aninstruction by a user. The turning and spherical aberration correctionpart 40 is provided in part of the optical system including theobjective lens actuator. The beam spot of the laser beam on themultilayer information recording medium 1 is optimized by driving andcontrolling the turning and spherical aberration correction part 40.

The objective lens actuator is driven and controlled by the focus driver41. A focus jump signal is supplied from the control part of theobjective lens actuator to the focus driver 41 based on an operationinstruction in order to reproduce information from a desired recordinglayer.

In the process of performing focus search according to this embodiment,the spherical aberration control circuit 13 determines an amount ofaberration correction by using such a method of generating a signal(spherical aberration signal) based on the difference between the focuspositions of the central region A and the peripheral region B of theliquid crystal element as described in the first through thirdembodiments. The spherical aberration control circuit 13 supplies theamount of aberration correction corresponding to the recording layerthat the information is to be recorded on or reproduced from to theturning and spherical aberration correction part 40 in the form ofapplication of given voltages.

According to the above-described configuration, in the case ofreproducing information from a desired recording layer of the multilayerinformation recording medium 1, an amount of aberration correctioncorresponding to the desired recording layer is detected and sphericalaberration correction is performed by applying given voltages based onthe detection signal. Further, while the turning part 7 and thespherical aberration correction part 12 are separately provided in theoptical pickup unit in each of the first and third embodiments, thefunctions of the turning part 7 and the spherical aberration correctionpart 12 are realized by the single unit of the turning and sphericalaberration correction part 40 having the liquid crystal having the twofunctions in the fourth embodiment. By thus integrating the turning part7 and the spherical aberration correction part 12 into the single unit,the cost of components can be reduced. The number of assembly steps isalso decreased so that further cost reduction can be realized.Furthermore, the optical pickup unit can be downsized.

FIG. 14 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a fifth embodiment of thepresent invention. The optical pickup unit of the fifth embodimentincludes a turning and spherical aberration correction part 50. In FIG.14, the same elements as those of FIGS. 5 and 10 are referred to by thesame numerals, and a description thereof will be omitted.

The turning and spherical aberration correction part 50 is composed of aliquid crystal element 50 a and the polarizing plate 8 for selectivelytransmitting or blocking off a reflected light from a recording layer ofthe multilayer information recording medium 1 for which recording layerthe optical characteristics of the objective lens 6 are not optimized.The turning and spherical aberration correction part 50 is provided inthe optical path from the beam splitter 4 and the light-receivingelement 10 as a replacement for the turning part 7 and the sphericalaberration correction part 12 of the first embodiment.

Transparent electrode patterns are formed concentrically on the liquidcrystal 50 a of the turning and spherical aberration correction part 50.Like the turning and spherical aberration correction part 40, theturning and spherical aberration correction part 50 is configured sothat such switching between the central region A and the sphericalregion B as illustrated in FIG. 6 and such selection of an amount ofreduction of the beam spot diameter as illustrated in FIGS. 8 and 9Athrough 9G can be performed by selecting electrode patterns to whichvoltages are to be applied.

First, an amount of spherical aberration for a recording layer of themultilayer information recording medium 1 for which layer the opticalcharacteristics of the objective lens 6 are not optimized is detected byswitching the voltages applied to the turning and spherical aberrationcorrection part 50 in accordance with the process as described in thefirst embodiment. Then, given voltages are applied to the concentricelectrode patterns based on the amount of spherical aberration so thatthe planes of polarization of light beams passing through the respectiveconcentric electrode patterns are different from each other. Forinstance, while a constant voltage is applied to the central region ofthe turning and spherical aberration correction part 50, a voltageapplied to the concentric spherical region thereof is changed so as tochange the optical phase of the spherical region.

Then, only a laser beam of a given phase is allowed to pass through thepolarizing plate 8 of the turning and spherical aberration correctionpart 50 to be received by the light-receiving element 10.

According to the above-described configuration, of a reflected lightfrom a recording layer for which the optical characteristics of theobjective lens 6 are not optimized, the peripheral part, which issignificantly affected by spherical aberration, is prevented from beingreceived by the light-receiving element 10. This prevents deteriorationof the quality of reproduced signals.

The present invention is not limited to the specifically disclosedembodiments of the optical pickup unit of the present invention. Forinstance, spherical aberration is detected by separately detecting thefocus positions of the central region A and the peripheral region Baccording to the spherical aberration method shown in theabove-described embodiments. However, spherical aberration may bedetected by separately detecting the focus position of the sum of thecentral region A and the peripheral region B and the focus position ofthe peripheral region B. Alternatively, the focus positions of aplurality of regions may be detected instead of those of the centralregion A and the peripheral region B. In such a case, a variety ofcombinations of the focus positions are available so that correction canbe made more precisely.

FIG. 15 is a perspective view of an information recording andreproduction apparatus according to a sixth embodiment of the presentinvention. The information recording and reproduction apparatus of FIG.15 includes an optical disk 60 that is a multilayer informationrecording medium, a cartridge 61 accommodating the optical disk 60, ashutter 62 provided to the cartridge 61 to be openable (and closable) sothat the recording surface may be exposed externally, a dust-proof case63 serving as the exterior of the information recording and reproductionapparatus, an opening part 64 formed on the dust-proof case 63 so thatthe cartridge 61 is inserted thereinto or extracted therefrom throughthe opening part 64, a spindle motor 65 rotating the optical disk 60, acarriage 66 provided with an optical pickup unit, and a carriage movingmechanism 67 moving the carriage 66 in the radial direction of theoptical disk 60.

In FIG. 15, a variety of signal processing circuits and input and outputterminals, which are practically necessary components in the informationrecording and reproduction apparatus, are not shown.

The optical pickup unit supported on the carriage 66 is any of theoptical pickup units of FIGS. 5 and 10 through 14. Information isrecorded on or reproduced from the optical disk 60 by this opticalpickup unit.

The information recording and reproduction apparatus is thus configuredso as to be capable of recording information on and reproducinginformation from a multilayer information recording medium.

FIG. 16 is a schematic diagram showing a configuration of an opticalsystem of an optical pickup unit according to a seventh embodiment ofthe present invention. The optical pickup unit of FIG. 16 includes asemiconductor laser 301 that is a light source, a collimator lens 302, afirst electro-optical element 303 having a transmittance change regionand an aberration correction region, a polarization beam splitter 304, adeflection mirror 305, a ¼ wave plate 306, an objective lens 307, asecond electro-optical element 309 having a transmittance change regionand an aberration detection signal generation region, a detection lens310, a light-receiving element 311 that is an element detecting a lightbeam, and a control circuit 312 controlling the first and secondelectro-optical elements 303 and 309.

As shown in FIG. 16, a light beam emitted from the semiconductor laser301 is converted into substantially parallel rays by the collimator lens302 to be incident on the first electro-optical element 303 providedbehind (subsequent to) the semiconductor laser 301 in the optical pathbetween the semiconductor laser 301 and an information recording medium308. As will be described later, the first electro-optical element 303is composed of a first region that is the transmittance change regionswitching a transmittance for recording information on the informationrecording medium 308 and a transmittance for reproducing informationtherefrom and a second region that is the aberration correction regionperforming aberration correction by providing the light beam passingtherethrough with a phase reverse to the wavefront aberration of thelight beam. The light beam passing through the first electro-opticalelement 303 passes through the polarization beam splitter 304 to haveits optical path deflected 90° by the deflection mirror 305. Thereafter,the light beam is collected by the objective lens 307. The ¼ wave plate306 is provided between the deflection mirror 305 and the objective lens307. The ¼ wave plate 306 converts the light beam from a linearlypolarized light to a circularly polarized light so that the light beamis focused onto the information recording medium 308 circularlypolarized.

The reflected light beam from the information recording medium 308travels through the optical path of the emitted light beam in theopposite direction to again reach the polarization beam splitter 304 viathe objective lens 307, the ¼ wave plate 306, and the deflection mirror305. The light beam reflected from the information recording medium 308to be incident on the ¼ wave plate is circularly polarized in a reversedirection compared with the light beam emitted from the ¼ wave plate 306toward the information recording medium 308. The reflected light beampasses through the ¼ wave plate 306 to be converted into a linearlypolarized light whose plane of polarization is perpendicular to that ofthe linearly polarized light beam emitted from the semiconductor laser301. The linearly polarized reflected light beam is reflected from thepolarization beam splitter 304, which transmits the light beam emittedfrom the semiconductor laser 301. The reflected light beam reflectedfrom the polarization beam splitter 304 is incident on the secondelectro-optical element 309 provided in front of (preceding) thelight-receiving element 311 in the optical path between the informationrecording medium 308 and the light-receiving element 311. As will bedescribed later, the second electro-optical element 309 is composed of athird region that is the transmittance change region changing itstransmittance at the time of recording and reproduction depending on thetype of the information recording medium 8 and a fourth region that isthe aberration detection signal generation region switching ON or OFFtransmittance for the light beam passing therethrough in order togenerate an aberration detection signal.

The light beam passing through the second electro-optical element 309passes through the detection lens 310 to reach the light-receivingelement 311. The light-receiving element is suitably divided inaccordance with a servo signal generation method. The reflected lightfrom the information recording medium 308 is detected by thelight-receiving element 311 to be output to other succeeding controlcircuits (not shown in the drawing) as a tracking signal, a focussignal, and a reproduction signal.

In the conventional optical pickup unit, at a time of recordinginformation on (writing information to) an information recording medium,the output of a semiconductor laser, which is set to a high level, ishardly affected by a returning light. On the other hand, at a time ofreproducing (reading) information from the information recording medium,the output of the semiconductor laser is set to a low level. Therefore,the output of the semiconductor laser is subject to the effect of thereturning light in the conventional optical pickup unit.

According to the configuration of the present invention, compared withthe conventional configuration, the output of the semiconductor laser301 can be increased at the time of reproduction so that thesemiconductor laser 301 can emit light with power a few milliwattsgreater than the threshold current level without changing the amount oflight reaching the information recording medium 308. By performing suchcontrol, the output of the semiconductor laser 301 can be set to a levelhigher than the conventional level at the time of reproduction, so thatthe semiconductor laser 301 is less affected by the returning light.Further, by setting the transmittance of the first electro-opticalelement 303 to a low rate (value), a rate of returning light to thesemiconductor laser 301 is reduced, so that the light emission of thesemiconductor laser 1 is stabilized and noise in the output of thesemiconductor laser 301 is reduced.

FIG. 17 is a sectional view of the first electro-optical element 303.The first electro-optical element 303 includes a pair of glasssubstrates 315 a and 315 b. Transparent electrodes 316 a and 316 b areformed of ITO (In₂O₃.SnO₂) on the glass substrates 315 a and 315 b,respectively. Polyimide alignment films 317 a and 317 b, which have beensubjected to alignment processing by rubbing, are formed on the ITOtransparent electrodes 316 a and 316 b, respectively.

A gap material (not shown in the drawing) is provided between the pairedglass substrates 315 a and 315 b so that the paired glass substrates 315a and 315 b oppose each other with a given distance therebetween. Aspace between the paired glass substrates 315 a and 315 b is sealed witha seal material (not shown in the drawing). A given liquid crystal issealed into the space so as to form a liquid crystal layer 318.

The first electro-optical element 303 is formed as a liquid crystalcell. In this liquid crystal cell, the alignment of liquid crystalmolecules in the liquid crystal layer 318 is continuously twisted 90°between the lower and upper alignment layers 317 a and 317 b whenalignment processing is performed so that the lower and upper alignmentfilms 317 a and 317 b are 90° different in directions in which thelongitudinal axes of liquid crystal molecules are aligned, that is, whenthe lower and upper alignment films 317 a and 317 b are rubbed indirections 90° different from each other. Further, as shown in FIG. 17,a polarizing film 319, for instance, is deposited on the liquid crystalcell on the side opposite to the semiconductor laser 301 opposing theliquid crystal cell so as to cover a region corresponding to thetransmittance change region. The polarizing film 319 is provided so thatits polarization axis coincides with the direction of alignment ofliquid crystal molecules on the surface of the adjacent glass substrate315 b.

According to this arrangement, a relationship between a voltage Vapplied to the liquid crystal cell and its transmittance T is as shownin FIG. 18. That is, when a low voltage is applied to the liquid crystalcell, the plane of polarization of the light beam emitted from thesemiconductor laser 301 to be incident on the liquid crystal cell isturned 90° in accordance with the twist of the liquid crystal moleculesto be absorbed into the polarizing film 319.

That is, in the case of applying a low voltage to the liquid crystalcell, transmittance for the light beam emitted from the semiconductorlaser 301 is controlled to a low rate (value). On the other hand, when ahigh voltage is applied to the liquid crystal cell, the twist of thealignment of the liquid crystal molecules disappears. As a result, thelight beam emitted from the semiconductor laser 301 and incident on theliquid crystal cell travels straight without its plane of polarizationbeing turned to pass through the polarizing film 319. That is, in thecase of applying a high voltage to the liquid crystal cell, the lightbeam emitted from the semiconductor laser 301 is controlled to a highrate (value).

In the case of employing this liquid crystal cell, which can performsuch transmittance control, as the first electro-optical element 303 inthe optical pickup unit, the light-emission power of the semiconductorlaser 301 is set to 35 mW and the transmittance of the firstelectro-optical element 303 is set to 85% at the time of recording, forinstance, so that the optical system may have a usability of light of40% with respect to the light beam traveling up to the informationrecording medium 308. Therefore, the recording power of the light beamcan be set to approximately 12 mW on the information recording medium308.

On the other hand, at the time of reproduction, the light-emission powerof the semiconductor laser 301 can be set to eight milliwatts with thetransmittance of the first electro-optical element 303 being set to alow rate, for instance, 30%. In this case, the reproduction power of thelight beam passing through the same optical system as at the time ofrecording can be set to approximately one milliwatt on the informationrecording medium 308.

As previously described, the semiconductor laser 301 is considerablyaffected by the returning light while its light-emission power is in therange up to approximately five milliwatts. According to the presentinvention, however, the semiconductor laser 301 is allowed to emit thelight beam with a sufficient light-emission power of, for instance,eight milliwatts at the time of reproduction. Therefore, noise effectscan be avoided without employing a high-frequency superimposed circuit.

As shown in FIG. 19, the electro-optical element 303 has the aberrationcorrection region for correcting wavefront aberration (the secondregion) provided outside the transmittance change region (the firstregion). In the second region, at least one of the transparentelectrodes 316 a and 316 b of the electro-optical element 303 is dividedaccording to a given pattern. A voltage applied to each dividedelectrode part is variably controlled based on the later-describedaberration detection signal so that the refractive index of each dividedpart is changed to provide a phase difference to a light beam (part ofthe light beam) passing through the divided part. Thereby, the wavefrontaberration including coma and spherical aberration of the objective lens307 can be corrected. That is, by controlling the voltages applied tothe divided second region, the refractive index n of the liquid crystalof each divided part of the second region can be varied freely from n1to n2.

The fact that the refractive index n is variable means that the lightbeam passing through each divided part of the second region can beprovided with an optical path difference Δn·d (Δn is a variation of therefractive index and d is a liquid crystal cell thickness), that is, aphase difference Δn·d(2π/λ) (λ is the wavelength of the light beam).Thus, by controlling the applied voltages in accordance with wavefrontaberration occurring in the objective lens 307 so as to change therefractive index n of each divided part of the second region, wavefrontaberration caused by the objective lens 307 can be corrected.

FIGS. 20A through 20C are diagrams showing typical types of wavefrontaberration. FIGS. 20A through 20C show spherical aberration, coma, andastigmatism, respectively. As is apparent from FIGS. 20A through 20C,the peripheral part of a light beam passing through an objective lens issignificantly affected by wavefront aberration, while a returning lightaffects the central region of the light beam, where the power of thelight beam is high. Thus, stable control of the semiconductor laser 301and correction of the wavefront aberration of the light beam incident onthe objective lens 307 can be performed compatibly by the singleelectro-optical element 303 by dividing the region thereof.

Further, in the optical pickup unit of FIG. 16, the intensity of thelight beam (signal light) incident on the light-receiving element 311 isextremely high at the time of recording information on the informationrecording medium 308 since the output power of the semiconductor laser301 is set to a high level. On the other hand, at the time ofreproducing information from the information recording medium 308, theoutput power of the semiconductor laser 301 is set to a low level sothat the intensity of the light beam is low. Recently, there have been aplurality of types of recording media that are employable as theinformation recording medium 308, such as a ROM medium, a write-oncemedium, and a phase-change medium. Reflectivity is different in eachtype of recording medium. Thus, an optimum signal level differsdepending on an operation (that is, whether recording or reproduction isperformed) or a medium type.

If the amplitude level of a signal is too low, a problem is caused ininformation reproduction. Therefore, the gain of a current-voltageconversion amplifier succeeding the light-receiving element 311 isswitchable so that the output amplitude of the current-voltageconversion amplifier falls within a proper range. However, there hasbeen a necessity of considering the type of the information recordingmedium 308 or a plurality of conditions of recording and reproduction.For instance, in the case of making the conventional gain switchingcircuit as shown in FIG. 4 suitable for a plurality of conditionpatterns, the number of resistors attached to the circuit increases sothat a signal response speed is decreased.

According to the present invention, at the time of reproduction, anamount of light reflected from the information recording medium 308remains unchanged from that in the conventional configuration because ofthe second electro-optical element 309, while at the time of recording,the transmittance of the second electro-optical element 309 for thelight beam reflected from the information recording medium 308 is set toa low rate. Thereby, the gain of the current-voltage conversionamplifier remains the same at the time of recording and at the time ofreproduction. Further, the dynamic range of the light-receiving element311 is restricted so that the output of the light-receiving element 311is saturated if the input level is too high. According to the presentinvention, an amount of light directed onto the light-receiving element311 itself can be controlled by the second electro-optical element 309so that restriction resulting from the dynamic range can be relaxed.

The second electro-optical element 309 has the same configuration as thefirst electro-optical element 303 shown in FIG. 17. Further, the secondelectro-optical element 309 has the aberration detection signalgeneration region (the fourth region) provided outside the transmittancechange region (the third region). The fourth region includes a shutterfunction for generation of the aberration detection signal. The fourthregion, which is a transmittance change part of the same configurationas the third region, has a transparent electrode divided according tothe aberration detection signal to be generated.

As shown in FIG. 21, which is a diagram showing occurrence of sphericalaberration, spherical aberration is a phenomenon that the focus positionof part of a light beam around the axis of its beam spot is differentfrom that of a peripheral part of the light beam. By separating the twoparts of the light beam and detecting the focus position of each of thetwo parts separately, an amount of spherical aberration can be obtainedfrom a difference between the focus positions. On the other hand, anamount of light to the light-receiving element 311 is required to becontrolled in the high-power central region of the light beam.Therefore, stable signal detection and generation of the aberrationdetection signal for aberration correction can be realized by the singleelement irrespective of the operation performed (recording orreproduction) and the medium type by dividing the region of the element.

A description will now be given of specific examples of aberrationcorrection and detection by an electro-optical element employed as thefirst and second electro-optical elements 303 and 309. Needless to say,transmittance for the light beam emitted from the light source(semiconductor laser 301) is changed in the optical path toward theinformation recording medium 308 (a lighting optical path) and theamount of light directed onto the light-receiving element 311 iscontrolled in the optical path from the information recording medium 308(a detection optical path).

If there is a variation in the thickness of the information recordingmedium 308, spherical aberration occurs when the light beam passesthrough the substrate of the information recording medium 308. In afirst example is shown a configuration of the electro-optical element inthe case of correcting this spherical aberration.

The spherical aberration is detected by the light-receiving element 311and the electro-optical element is actuated to cancel this sphericalaberration, so that the spherical aberration is corrected. FIG. 22A is adiagram showing wavefront aberration when spherical aberration occurs.The wave surface of the light beam includes delays 23 a and 23 b withrespect to a reference wave surface 22. The delays 23 a and 23 b occursymmetrically with respect to an optical axis 21. When the referencewave surface 22 is focused, a position at which the delayed wave surfaceis focused is a defocus with respect to the focus point of the referencewave surface 22.

Therefore, the occurrence of the spherical aberration can be understoodby detecting a focus condition by obtaining a difference between thedelayed wave surface and the reference wave surface 22. For instance, ifthe electro-optical element has a pattern shown in FIG. 22B, theaberration detection signal of the spherical aberration can be generatedbased on the signal of the light-receiving element 311 obtained bytransmitting the light beam selectively through an A region 24 and a Bregion 25.

Next, a description will be given of a configuration for correctingspherical aberration by the electro-optical element based on theaberration detection signal. FIG. 23A is a diagram showing a section ofthe spherical aberration of FIG. 20A. As shown in FIG. 20A, thespherical aberration becomes greater in proportion to the distance fromthe optical axis. Therefore, the spherical aberration can be canceled bythe electro-optical element (liquid crystal cell) providing the lightbeam passing therethrough with a phase difference indicated by a brokenline in FIG. 23A, which phase difference is reverse to that indicated bythe solid line in FIG. 23A. FIG. 23B is a diagram showing the sphericalaberration after correction, which is the sum of the solid and brokenlines of FIG. 23A. FIG. 23B shows that the spherical aberration isconsiderably reduced compared with its original amount. FIG. 24A is adiagram showing a pattern for correcting the spherical aberration in theelectro-optical element.

If the information recording medium 308 is inclined to have a tilt, comais generated when the light beam passes through the substrate of theinformation recording medium 308. In a second example is shown aconfiguration of the electro-optical element in the-case of correctingthis coma.

The coma is detected by the light-receiving element 311 so that theelectro-optical element is driven to cancel the coma. Thereby, the comais corrected. FIG. 25A is a diagram showing wavefront aberration whenthe coma occurs. The wave surface of the light beam includes an advance26 a and a delay 26 b with respect to its reference wave surface 22.When the reference wave surface 22 is focused, a position at which eachof the advanced and delayed wave surfaces is focused is a defocus withrespect to the focus point of the reference wave surface 22.

Therefore, the occurrence of the coma can be understood by detecting afocus condition by obtaining a difference between each of the advancedand delayed wave surfaces and the reference wave surface 22. Forinstance, if the electro-optical element has a pattern shown in FIG.25B, the aberration detection signal of the coma can be generated basedon the signal of the light-receiving element 311 obtained bytransmitting the light beam selectively through an A₁ region 27 and a B₁region 29, and an A₂ region 28 and a B₂ region 30.

FIG. 26A is a diagram showing a section of the coma caused by the tilt.In this case as well, the coma due to the tilt can be canceled as shownin FIG. 26B by the electro-optical element (liquid crystal cell)providing the light beam passing therethrough with a phase differenceindicated by a broken line in FIG. 26A. FIG. 24B is a diagram showing apattern for correcting the coma in the electro-optical element.

When the light beam passes through the substrate of the informationrecording medium 308, astigmatism is generated due to double refractioncaused by the information recording medium 308. In a third example isshown a configuration of the electro-optical element for correcting thisastigmatism.

The astigmatism is detected by the light-receiving element 311 so thatthe electro-optical element is driven to cancel the astigmatism.Thereby, the astigmatism is corrected. Detection of the astigmatism canbe performed based on the same idea as detection of the above-describedspherical aberration and coma. FIG. 24C is a diagram showing a patternfor correcting the astigmatism in the electro-optical element.

Further, a plurality of types of aberration can be eliminated at thesame time by combining the patterns of FIG. 24A through 24C.Furthermore, the above-described electro-optical element may beincorporated into the first and second electro-optical elements 303 and309 of FIG. 16 as the second and fourth regions thereof.

The optical pickup unit of the seventh embodiment of the presentinvention can be provided in the information recording and reproductionapparatus of FIG. 15. That is, an information recording and reproductionapparatus according to the seventh embodiment of the present inventioncan be realized by providing the optical pickup unit of the seventhembodiment in the information recording and reproduction apparatus ofFIG. 15.

The present invention is not limited to the specifically disclosedembodiments, but variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority patentapplications No. 2001-178502filed on Jun. 13, 2001 and No. 2001-371079filed on Dec. 5, 2001, the entire contents of which are herebyincorporated by reference.

1. An optical pickup unit performing recording or reproducinginformation by making a light beam emitted from a light source incidenton an information recording medium, the optical pickup unit comprising:an electro-optical element switching values of transmittance of a givenregion through which the light beam passes depending on whether theinformation is recorded or reproduced, and wherein said electro-opticalelement is provided subsequent to the light source in an optical pathbetween the light source and the information recording medium; and thetransmittance of the given region of said electro-optical element iscontrolled to a first value at a time of reproducing the informationfrom the information recording medium and to a second value at a time ofrecording the information on the information recording medium, the firstvalue being smaller than the second value, and the electro-opticalelement further comprising an outer region provided outside the givenregion so that the light beam passing through the outer region has aphase difference thereof selectively varied in the outer region, whereinthe outer region includes a transparent electrode divided into aplurality of parts; and the phase difference of the light beam passingthrough the outer region is selectively varied therein by controllingvoltages applied to the divided parts so as to vary refractive indicesthereof.
 2. An information recording and reproduction apparatusrecording information on and reproducing information from an informationrecording medium, the information recording and reproduction apparatuscomprising: an optical pickup unit performing recording or reproducinginformation by making a light beam emitted from a light source incidenton the information recording medium, the optical pickup unit comprising:an electro-optical element switching values of transmittance of a givenregion through which the light beam passes depending on whether theinformation is recorded or reproduced, and wherein said electro-opticalelement is provided subsequent to the light source in an optical pathbetween the light source and the information recording medium; and thetransmittance of the given region of said electro-optical element iscontrolled to a first value at a time of reproducing the informationfrom the information recording medium and to a second value at a time ofrecording the information on the information recording medium, the firstvalue being smaller than the second value, and the electro-opticalelement further comprising an outer region provided outside the givenregion so that the light beam passing through the outer region has aphase difference thereof selectively varied in the outer region, whereinthe outer region includes a transparent electrode divided into aplurality of parts; and the phase difference of the light beam passingthrough the outer region is selectively varied therein by controllingvoltages applied to the divided parts so as to vary refractive indicesthereof.